Code generation build plugin

Package.swift

@@ -26,6 +26,10 @@ let products: [Product] = [
     name: "protoc-gen-grpc-swift",
     targets: ["protoc-gen-grpc-swift"]
   ),
+  .plugin(
+    name: "GRPCProtobufGenerator",
+    targets: ["GRPCProtobufGenerator"]
+  ),
 ]
 
 let dependencies: [Package.Dependency] = [
@@ -101,6 +105,16 @@ let targets: [Target] = [
     ],
     swiftSettings: defaultSwiftSettings
   ),
+
+  // Code generator build plugin
+  .plugin(
+    name: "GRPCProtobufGenerator",
+    capability: .buildTool(),
+    dependencies: [
+      .target(name: "protoc-gen-grpc-swift"),
+      .product(name: "protoc-gen-swift", package: "swift-protobuf"),
+    ]
+  ),
 ]
 
 let package = Package(

Plugins/GRPCProtobufGenerator/BuildPluginConfig.swift

@@ -0,0 +1,127 @@
+/*
+ * Copyright 2024, gRPC Authors All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import Foundation
+
+let configFileName = "grpc-swift-proto-generator-config.json"
+
+/// The configuration of the build plugin.
+struct BuildPluginConfig: Codable {
+  /// The visibility of the generated files.
+  ///
+  /// Defaults to `Internal`.
+  var visibility: GenerationConfig.Visibility
+  /// Whether server code is generated.
+  ///
+  /// Defaults to `true`.
+  var server: Bool
+  /// Whether client code is generated.
+  ///
+  /// Defaults to `true`.
+  var client: Bool
+  /// Whether message code is generated.
+  ///
+  /// Defaults to `true`.
+  var message: Bool
+  /// Whether imports should have explicit access levels.
+  ///
+  /// Defaults to `false`.
+  var useAccessLevelOnImports: Bool
+
+  /// Specify the directory in which to search for imports.
+  ///
+  /// Paths are relative to the location of the specifying config file.
+  /// Build plugins only have access to files within the target's source directory.
+  /// May be specified multiple times; directories will be searched in order.
+  /// The target source directory is always appended
+  /// to the import paths.
+  var importPaths: [String]
+
+  /// The path to the `protoc` binary.
+  ///
+  /// If this is not set, Swift Package Manager will try to find the tool itself.
+  var protocPath: String?
+
+  // Codable conformance with defaults
+  enum CodingKeys: String, CodingKey {
+    case visibility
+    case server
+    case client
+    case message
+    case useAccessLevelOnImports
+    case importPaths
+    case protocPath
+  }
+
+  let defaultVisibility: GenerationConfig.Visibility = .internal
+  let defaultServer = true
+  let defaultClient = true
+  let defaultMessage = true
+  let defaultUseAccessLevelOnImports = false
+  let defaultImportPaths: [String] = []
+
+  init(from decoder: any Decoder) throws {
+    let container = try decoder.container(keyedBy: CodingKeys.self)
+
+    self.visibility =
+      try container.decodeIfPresent(GenerationConfig.Visibility.self, forKey: .visibility)
+      ?? defaultVisibility
+    self.server = try container.decodeIfPresent(Bool.self, forKey: .server) ?? defaultServer
+    self.client = try container.decodeIfPresent(Bool.self, forKey: .client) ?? defaultClient
+    self.message = try container.decodeIfPresent(Bool.self, forKey: .message) ?? defaultMessage
+    self.useAccessLevelOnImports =
+      try container.decodeIfPresent(Bool.self, forKey: .useAccessLevelOnImports)
+      ?? defaultUseAccessLevelOnImports
+    self.importPaths =
+      try container.decodeIfPresent([String].self, forKey: .importPaths) ?? defaultImportPaths
+    self.protocPath = try container.decodeIfPresent(String.self, forKey: .protocPath)
+  }
+}
+
+extension GenerationConfig {
+  init(configurationFile: BuildPluginConfig, configurationFilePath: URL, outputPath: URL) {
+    self.visibility = configurationFile.visibility
+    self.server = configurationFile.server
+    self.client = configurationFile.client
+    self.message = configurationFile.message
+    // hard-code full-path to avoid collisions since this goes into a temporary directory anyway
+    self.fileNaming = .fullPath
+    self.useAccessLevelOnImports = configurationFile.useAccessLevelOnImports
+    self.importPaths = []
+
+    // Generate absolute paths for the imports relative to the config file in which they are specified
+    self.importPaths = configurationFile.importPaths.map { relativePath in
+      configurationFilePath.deletingLastPathComponent().relativePath + "/" + relativePath
+    }
+    self.protocPath = configurationFile.protocPath
+    self.outputPath = outputPath.relativePath
+  }
+}
+
+extension GenerationConfig.Visibility: Codable {
+  init?(rawValue: String) {
+    switch rawValue.lowercased() {
+    case "internal":
+      self = .internal
+    case "public":
+      self = .public
+    case "package":
+      self = .package
+    default:
+      return nil
+    }
+  }
+}